Gas turbine power plants produce electric power by combusting fuel and compressed air in a combustion chamber and then using the resulting high temperature, high pressure combustion gas to rotate an expander which drives a generator to produce electric power. In general, these turbines include a compression section for compressing air entering the turbine, a combustion section following the compression section for combusting the fuel with compressed air, and an expansion section after the combustion section where the combustion gas from the combustion section is expanded to generate shaft work. The shaft work is transferred to an electrical generator that converts the shaft work into electricity.
Gas turbines operate based upon the Brayton cycle in three phases. First, work is performed on the air by compressing the air isentropically in the compression section. Heat is then added to the compressed air isobarically in the combustion section. The hot compressed air is then isentropically expanded down to a lower pressure in the expansion section. The Brayton cycle has inherent inefficiencies because much of the energy imparted to the air during the compression and heating remains in the relatively high temperature, low pressure exhaust gas exiting the expansion section. In many cases, the exhaust gas temperature may approach or exceed 1000 F. (538 Degrees C.). If vented to the atmosphere, the portion of the combustion fuel used to raise the exhaust gas to this temperature is wasted, resulting in poor overall cycle efficiency.
Recovering the remaining energy in the expander exhaust gas improves the overall efficiency of such plants. In one known approach, the expander exhaust gas is used to produce steam, which is then used to produce additional electric power in a condensing steam turbine. Thermodynamically, condensing steam to make power is inefficient because about two thirds of the energy is lost to cooling water in the condensing cycle and only about one third of the energy is converted to electricity. Because steam turbines operate on the Rankine cycle, and not the Brayton cycle, plants utilizing this heat recovery method are known as combined cycle plants.
Notwithstanding this loss of energy, the development of large advanced gas turbines has resulted in a substantial reduction in the capital investment required to install combined cycle power plants, and a significant increase in their efficiency. A useful measure of cycle efficiency is known as the heat rate, defined for combined cycle plants as the latent heat valve of the fuel consumed (BTU/H) divided by power produced (KW). Combined cycle plants using modern gas turbines can now produce power for less than 7000 BTU/KWH.
These advanced gas turbines use higher combustion temperatures and compression ratios to convert more of the combustion fuel directly to electric power in the expander. The amount of energy in the exhaust per unit of electric power produced by the expander is reduced, and hence the amount of energy lost to cooling water in the condensing steam turbine is also reduced. This translates to fuel savings and a lower heat rate. Advanced gas turbines also generate less exhaust gas per unit of power production at a higher exhaust temperature. Because the amount of exhaust gas is less and at higher temperature, a larger percentage of the exhaust energy can be recovered by the condensing steam turbine also leading to higher efficiency and a lower heat rate.
In a cogeneration power plant, the thermal energy of the expander exhaust is used to generate steam or some other heating medium such as hot oil, the net products being electric power and the cogenerated products (steam or another heating medium). The amount of thermal energy that can be absorbed by the steam generator or hot oil heater is referred to herein as the available heat sink. Large advanced gas turbines are often unsuitable in cogeneration applications because they require a very large available heat sink due to their large throughput and the high exhaust temperature.
For example, if the thermal energy load of the expander exhaust gas would generate more steam than is required for use elsewhere in the plant as a heating medium or otherwise, the exhaust energy load exceeds the available heat sink. Such limitations in the available heat sink may limit the size of the gas turbine in a cogeneration plant and prevent exploitation of the resulting economies of scale and efficiencies associated with larger turbines.
As a result, cogeneration power plants have become increasingly difficult to justify economically. Advances in gas turbines have made the difference in efficiency between combined cycle and cogeneration plants relatively small. The installation cost per kilowatt of a combined cycle plant is now considerably less than a cogeneration plant because of the economy of scale associated with the use of larger turbines in combined cycle plants.
With further advances in the size and efficiency of advanced gas turbines already on the horizon, a new heat recovery design is required to keep cogeneration plants viable, and to permit the efficient use of large gas turbines in cogeneration applications. Because cogeneration plants are more efficient than combined cycle plants and burn less fuel per kilowatt at a given turbine size, there are also environmental incentives for keeping cogeneration plants viable.
Conventional cogeneration power plants are also generally designed to be run base loaded, i.e., at their maximum fuel and air throughput. Turning down the plant from its base load reduces the efficiency of electric power production. However, because electric power prices fluctuate with market demand, it may be desirable to turn down the plant from based loaded when power prices are low. Accordingly, a method which allows a cogeneration power plant to be run efficiently in a turndown condition is also required.
On the other hand, cogeneration power plants typically have little additional electric power available when power demand and prices are high. A method which permits power production to be increased during peak periods is also required to enable cogeneration power plants to remain economically viable.
Accordingly, it is an object of the present invention to provide a method for recovering heat from the exhaust gas of a gas turbine and utilizing such recovery of heat to reduce the heat sink requirement of the turbine. Such method should allow for variations in both the heat load of the exhaust gas and the available heat sink associated with steam and other heating medium requirements elsewhere in the plant.
It is another object of the invention to provide a method of efficiently increasing the peak power capacity of the gas turbine plant while reducing the heat sink requirement.
It is another object of the invention to provide a heat recovery scheme that allows efficient turndown operation during periods of diminished electric power demand and less favorable market conditions.
Briefly, these and other objects are accomplished by the invention, which is directed to an efficient method and apparatus for generating power in a cogeneration gas turbine power plant that overcomes the limitations of using large advanced gas turbines in cogeneration applications. The invention utilizes direct heat recovery from high temperature expander exhaust gas to increase cycle efficiency and reduce the heat sink requirements for cogeneration applications, particularly those employing large gas turbines. In one operational mode, the method involves adding water to increase the mass flow through the expander in order to increase power production. In another mode, the method permits power production to be reduced without a substantial loss in efficiency. The method and apparatus of the invention provides increased flexibility of operation, thus permitting the efficient production of peaking power during periods of high demand when it is most economically attractive, while also allowing efficient power production in a turndown condition when power prices are low. Thus, the method allows power production and production of the cogenerated product to be continually optimized based on current power market conditions and plant steam, heating oil, or other heat requirements.
Contrary to previously known cogeneration plants in which the gas turbine size was limited by the available heat sink for the exhaust gas thermal load, the method of the invention allows the exhaust thermal load to exceed the available heat sink while utilizing the high temperature exhaust characteristics of advanced gas turbines to improve cycle efficiency. The high temperature exhaust preheats the air entering the combustor, and in an important aspect, preheats compressed air exiting an air compressor before the compressed air enters the combustor. Every unit of thermal energy exchanged between the expander exhaust and combustion air saves a unit of combustion fuel. About a third of the gas turbine exhaust energy can be directly recovered in this manner, depending on operating conditions and equipment design. This method of recovering thermal energy from the expander exhaust is much more efficient than in a combined cycle plant, where about two thirds of the exhaust energy is lost to cooling water in the steam condenser. The resulting cooled exhaust gas is still hot enough to produce a cogenerated product downstream to recover additional thermal energy. Because there is less thermal energy remaining in the cooled expander exhaust, the required heat sink is also significantly reduced compared to current cogeneration heat recovery schemes where all the exhaust energy is used to produce steam.
In an important aspect, the method of the invention also includes saturating the preheated combustion air with water prior to combustion to produce peaking power. Saturating the combustion air provides greater mass flow through the expander, and has the potential for increasing power production significantly during peak power periods. Saturation of the combustion air also lowers the combustion flame temperature, resulting in lower NOx gas emissions. Thermal energy in the expander exhaust gas can also be used to heat the water used in the saturator. Preheating the combustion air allows it to carry more water without undesirable condensing of water in the combustion chamber. Since the capital cost for this additional power is low compared to the cost of installing additional gas turbines to meet peak power requirements, the method provides additional revenues and economic incentives for installing a cogeneration plant.
Thus, the method and apparatus of the invention allows producers of electric power to circumvent previous heat sink limitations on gas turbine size in cogeneration plants and thereby achieve increased operating efficiency and economies of scale, while also permitting greater flexibility to adjust operations to maximize profitability in changing market conditions.